bus-virt-phys-mapping.txt: standardize document format

Each text file under Documentation follows a different
format. Some doesn't even have titles!

Change its representation to follow the adopted standard,
using ReST markups for it to be parseable by Sphinx:

- Move author info to the beginning of file and use :Author:
- use warning/note annotation;
- mark literal blocks as such;
- Add a title for the document;
- use **emphasis** instead of _emphasis_.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>